Comparison of normal and tumor tissues show qualitative and quantitative changes in a number of enzyme systems. The steady state concentration of any enzyme in a tissue is a function of the rate of synthesis and the rate of degradation. To date the focus of attention has been almost entirely on the mechanisms involved in controlling the rate of synthesis. The rate of degradation is, however, equally important. We will determine the rate of degradation for the isozymes of the glycolytic enzyme, pyruvate kinase in normal rat liver and a series of Morris hepatomas. The Morris hepatomas are a series of tumors which differ in their growth rate and morphology, however, each tumor line retains a characteristic growth rate and morphology. The distribu-tion of the pyruvate kinase isozymes has been shown to change as a function of growth rate in the Morris hepatomas. This study of the rate of degradation of the pyruvate kinase isozymes in the Morris hepatomas therefore provides the opportunity to examine the role of this process in the cellular changes which are characteristic of neoplasia.